The downing rod is an Y-shaped tool designed to detect the presence of large quantities of ink. It is based on the principle of the dowsing rod, but is held at the other end and built with scientific backing. It works at least partially with GPS technology. The most likely candidate for having built it is Gyro Gearloose, but Honker Muddlefoot finished it.
Boom! Studios comics
Honker, as leader of the group tasked with finding the basin of ink present somewhere in St. Canard, holds the responsiblity for the downing rod. The device is not yet complete, as it lacks a component to establish satellite connection. That's why Honker's first goal is to find the gadget a part like that, which comes in the form of an abandoned cell phone. With the downing rod completed, the group is led into the caverns underneath St. Canard where they find an entire lake of the dangerous black substance.